1. Field of the Invention
This invention generally relates to temperature measurement in molten metals. The apparatus of the invention is particularly suitable for measurement of temperatures in the float forming chamber for forming flat glass.
2. Discussion of the Prior Art
The following references are considered pertinent to the invention in this application.
U.S. Pat. No. 3,816,183 to Krause discloses a temperature measuring device for repeated immersion in molten metal. The immersion end includes a vacuum cast sleeve of refractory fibers.
U.S. Pat. No. 3,329,766 to Cole discloses an expendable immersion thermocouple for measuring the temperature of molten metal. The thermocouple of Cole is designed for utilization in a steel bath of about 2,900.degree. F. The thermocouple is designed to survive the elevated temperatures for a time sufficient to made a temperature measurement. It is not designed for a long life.
U.S. Pat. No. 3,374,122 to Cole discloses an expendable immersion thermocouple including a weight. The thermocouple of Cole is designed to be weighted sufficient to drop through the slag at the top of a steel furnace without damage to the thermocouple during passage through the slag.
U.S. Pat. No. 2,019,695 to Ross discloses a thermocouple for temperature measurement in a bath of molten metal comprising a thermocouple protected by a carbon member and wrapped with an asbestos coating which has been impregnated with sodium silicate.
In the processes of float glass formation wherein molten glass is fomed into sheets on a pool of molten metal there has been a continuing interest in monitoring the temperature of the molten metal. Temperature variations of the molten metal at various longitudinal and transverse locations on the bath can affect the uniformity of the glass formed as portions of the glass resting on hotter tin will have lower viscosity and stretch by a greater amount than those which are resting on cooler portions of tin. It has been proposed that thermocouples be embedded in refractories forming the bottom of the float forming chamber. However, this has not been successfully accomplished. As for reasons not entirely clear, such thermocouples have been operable for only short periods of up to several months. In contrast the forming chamber bottoms themselves are utilized for periods of seven years or longer without reconstruction. Therefore, once the thermocouples embedded in the bottom of the bath become inoperable, they cannot be replaced. It is theorized that the thermocouples embedded in the refractory bottom of the bath fail due to any of the following factors or combinations of them: shifting of the bath refractories as they expand during heat up of the bath leading to physical fracture of the thermocouple, attack by hydrogen migrating through the bottom or attack by tin migrating through the bottom refractories.
It has also been proposed that thermocouples be inserted in the tin under the glass. However, thermocouples inserted in the tin have had a short life as the tin itself and the hydrogen forming the bath atmosphere and dissolved in the tin comprise a very corrosive environment leading to quick failure of the thermocouples. It is not practical to insert and then withdraw thermocouples to obtain readings under the glass in the tin bath as such insertion and withdrawal may set up undesirable flows in the tin and further the flows caused by insertion lead to inaccurate readings.
There remains a continuing need for a temperature sensing device which may be utilized to sense the temperature of the tin bath in the areas below the floating glass. There is a need for a device that will provide continuous monitoring of the tin bath to determine the effect of changes in throughput of the glass through the forming chamber or in the temperature of the glass delivered to the forming chamber. Further, there is a need to determine the effect on the tin bath of the movement of the tin by linear motors, effect of dams or barriers within the bath and the effect on tin temperature of overhead cooling or heating of the glass in the bath. The only temperatures currently available on a continuous basis are edge temperatures which are obtained by graphite protected rigid thermocouples in the edges of the bath, not in the area where the glass is located.